Abstract
Two-phase flow regimes were experimentally investigated during the entire condensation process of refrigerant R152a in a circular glass minichannel. The inner and outer diameters of the test minichannel were 0.75 and 1.50 mm. The channel was 500 mm long to allow observation of all the two-phase flow regimes during the condensation process. The experiments used saturation temperatures from 30 to 50°C, a mass flux of 150 kg/(m2·s) and vapor qualities from 0 to 1. The annular, intermittent and bubbly flow regimes were observed for the experimental conditions in the study. The absence of the stratified flow regime shows that the gravitational effect is no longer dominant in the minichannel for these conditions. Vapor-liquid interfacial waves, liquid bridge formation and vapor core breakage were observed in the minichannel. Quantitative measurements of flow regime transition locations were carried out in the present study. The experiments also showed the effects of the saturation temperature and the cooling water mass flow rate on flow regime transitions. The results show that the annular flow range decreases and the intermittent and bubbly flow ranges change little with increasing saturation temperature. The cooling water mass flow rate ranging from 38.3 kg/h to 113.8 kg/h had little effect on the flow regime transitions.
Highlights
Mini/Microchannels are widely used in many fields due to the advantages of enhanced heat transfer
Single-phase verification experiments show that the experimental apparatus is reliable and can be used for two-phase flow and condensation heat transfer experiments
The actual ranges of the intermittent and bubbly flow regimes change little and the proportions increase with increasing saturation temperature
Summary
Mini/Microchannels are widely used in many fields due to the advantages of enhanced heat transfer. The minichannels improve the system efficiency by reducing the air-side pressure drop and increasing the in-tube heat transfer coefficients. The minichannel heat exchangers have the advantages of a smaller refrigerant charge, which leads to lower greenhouse gas emissions. Channels with hydraulic diameters ranging from 0.2 to 3 mm are defined as minichannels by Kandlikar and Grande (2003) for single-phase and two-phase fluid flows. The test channel used in this study with a hydraulic diameter of 0.75 mm is classified as a minichannel. The flow regimes in Abbreviations: D, tube diameter, mm; Dh, hydraulic diameter, mm; hlv, latent heat, J/kg; ts, saturation temperature, oC; ps, saturation pressure, MPa; Z, flow regime coordinate, cm; Zc, length of the entire condensation process, cm Greek symbols; α, aspect ratio; λ, thermal conductivity, W/(m·K); μ, viscosity, Pa·s; σ, surface tension, N/m
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